North American Mesoscale Model Verification on January 26-27...

The North American Mesoscale Model (NAM) is a high resolution model courtesy of the National Center for Environmental Prediction. Another very common and referenced name for the model is the Weather Research and Forecasting Model (WRF). They are the same model and run out to 84 hours. The purpose of the model is to run a higher resolution determining mesoscale features that often cannot be picked up by the global models due to their larger scale. The NAM can be run on an 80km resolution with a broad view of the United States, or 40km which allows the viewer to zoom in on a localized region to put up on mesoscale features. Also a very high resolution form of the NAM exists using a 12km parametric and can zoom into different states. The model is released four times a day at 0z, 6z, 12z, and 18z. Keep in mind that time is zulu or more commonly known as greenwich mean time.

Computer models are critical to forecasting the weather as they use parametric and other mathematical equations to derive the current and predicted state of the atmosphere using a physical and chemical explanation as the background for the predictions. The NCEP is associated with the National Oceanic Atmospheric Administration (NOAA) to be the developer of these computer models. Each meteorological organization worlwide uses their own developed computer models to serve as a basis for the forecasts. Without computer models, we are not able to derive predictions on the weather greater than 24 hours out.

Like all computer models, the NAM shows all layers of the atmosphere in maps determining positioning and speed of the jet stream, surface precipitation amounts (quantitive precipitation forecast QPF), temperatures throughout the entire core of the atmosphere, convective indices, simulated radars, etc. Due to the higher resolution of the NAM, several problems do exist. It often produces convective elements in many middle latitude cyclones and these are known as convective feedback problems. This is where it developed these unstable regions and often associates them as surface lows causing the surface depiction to be distorted. This was a problem in the recent prediction of the January 26-27 winter storm and will be noted below. Also the NAM has a tendency to produce higher than normal precipitation amounts. I typically find myself cutting totals nearly by 30% as shown on the NAM. Finally the NAM has a bias to overamplify a low pressure and close them off to early particularily on eastern United States cyclogenesis off the coast.

The January 26-27 winter storm featured many model headaches do to some interesting variables. Early on it appeared likely for a low pressure to track up inland along the coastal plain. There was a lacking 50/50 low, unfavorable position of the western trough axis, neutral NAO, natural baroclinicity along the coastline, and stale antecedent cold air mass. This would produce rain along I-95 with heavy snows inland. The 500mb synoptic pattern showed very strong signals for this time of setup with most computer model guidance also in support. The GGEM and ECMWF led the pack with the heavy snows from I-81 on westward. The GFS suffered major problems with varying solutions for each run. But then the computer models began to delay the storm. In fact it was delayed nearly 48-60 hours from the original starting time. This caused a different scenario to unfold. The lacking high pressure to the north was still a problem as the anticyclone zoomed east-northeast, but now a high pressure and associated shortwave out ahead of it allowed sunk east-southeast across the Midwest and western Great Lakes. This acted as a 'kicker' helping to push the cyclonegenesis farther off the coast. This in turn allowed for a colder scenario along with precipitation to occur farther east. Therefore a turn of events allowed for a major I-95 snowstorm with 6in+ totals from Washington DC to Boston. This was very fortunate for snow lovers in that corridor due to pure luck given the poor synoptic setup.

Given the NAM's high resolution, it often overanalyzes prognostics post hour 60. It tends to enhance QPF, overamplify lows, and pick up on mesoscale features that really do not exist. Therefore I typically throw out hours 60-84. It would be a rare event where you would find the NAM 84 hour verifying anywhere close to accurate. But in the near term range, the NAM does an excellent job locating temperature thermals, QPF ranges, and picking up on mesoscale features; coastal fronts, enhanced convection, deformation bands, etc. But in this recent storm, it suffered a plethora of problems and even the 6 and 12 hour surface maps had poor verification especially in the QPF department.

Let us first look at the actual accumulated precipitation totals for the storm.Given this is a 24 hour accumulated precipitation amount, about .01-.1in of additional precipitation fell south of the Mason-Dixon line in the previous 24 hours.

Here are the preceeding NAM total QPF forecasts...

(January 24; 18z) (January 25; 6z)

(January 25; 12z) (January 25; 18z)

(January 26; 0z) (January 26; 6z)

As you can see, the NAM had a lot of variance with the northwestern sharp precipitation gradients. These tight gradients this year have been caused by the rapid intensification of the coastal lows allowing the heaviest moisture to be confined closer to the center of circulation. Also in this instance, a very cold and dry air mass along with associated cold front was quickly advancing southeast across the Great Lakes and was even picked up on by the 700mb RH charts. This allowed the flow out of the northwest to dry up some moisture for areas more inland.

The NAM simulated radar vs. the forecast QPF did not match up. Often the NAM simulated radar showed the heavier mesoscale bands lining up in northern Maryland and southern Pennsylvania where as it only showed total QPF to be .25in-.5in. In fact looking at total verification, the NAM did very well for its simulated radar.

(Actual NEXRAD National Radar) (6z NAM January 26 Simulated Radar)

The NAM did seem to have a hold on the 500mb map showing the negatively tilted trough producing the coastal low along with the placement of the upper level low and associated shortwave kicker just to the west.

(18z NAM January 24; 500mb) (0z NAM January 26; 500mb)

In general the differences in the 500mb maps were very subtle with just a general strengthening in the closed 500mb low, which verified a tad north of the January 24 18z model run.

The NAM did a very excellent job in identifying mesoscale band using the UUV/700mb RH charts.(6z NAM January 26; 700mb)

It indentifed the enhanced snow growth over southeastern Pennsylvania up through New Jersey and New York City. The problems with the NAM generally existed in the QPF fields. The model likely suffered a very convective feedback issues in QPF totals. This is why it is important to note other maps than surface maps to help locate the heaviest precipitation. The 700mb map screamed that snow totals would be farther inland with the enhanced deformation band and UUV rates. In general the high resolution models handled this the best with the HRR scoring an amazing victory for QPF along with the ECMWF. The GFS/NAM did a very poor job for QPF, but as noted above QPF does not always tell the story. Sometimes it is important to note other layers of the atmosphere to help make a forecast. This point is why many forecasters missed the boat. Many forecasters (especially broadcoast meteorologists) are drawn to the easy to understand QPF/surface maps, but one has to look at all layers of the atmosphere to make a prediction.

.NOW...PATCHY RAIN AND FREEZING RAIN WILL GIVE WAY TO A BRIEF BUTWIDESPREAD BURST OF SNOW THIS EVENING ACROSS THE SUSQUEHANNAVALLEY. THE LEADING EDGE OF THE SNOW WAS NEAR A MANSFIELD...JERSEY SHORE...CHAMBERSBURG LINE AT 540 PM. THE PRECIPITATIONIS MOVING NORTHEAST AT 45 MPH. THE AREA OF PRECIPITATION WILLBE EAST OF THE SUSQUEHANNA VALLEY BY 1030 PM. WHILE LESS ANINCH OF SNOW IS EXPECTED...THE COMBINATION OF FALLING TEMPERATURESAND GUSTY WINDS LATER THIS EVENING WILL MAKE FOR ICY SPOTS.

Quoting sunnysmum:Here in Brunswick, ME (southern/midcoast) it is snowing giant wet flakes. Snow started about 45 minutes and has accumulated about a half inch. Local weather forecast says it will turn to sleet and/or rain soon and then possibly back to snow by morning. Depending on which station you ask, we are looking at 1-3 or 3-6 when all is said and done. Hoping for 6 :)Actually it is already starting to "tick" when it hits the window... sigh...Temp is 34 degrees

Here in Brunswick, ME (southern/midcoast) it is snowing giant wet flakes. Snow started about 45 minutes and has accumulated about a half inch. Local weather forecast says it will turn to sleet and/or rain soon and then possibly back to snow by morning. Depending on which station you ask, we are looking at 1-3 or 3-6 when all is said and done. Hoping for 6 :)Actually it is already starting to "tick" when it hits the window... sigh...Temp is 34 degrees

Quoting TheRasberryPatch:temp is 33.2F. evergreens are filling up with ice, but all other trees look better than this morning. matter of fact i don't see ice on hardwood trees.

i am concerned with the sun going down are the temps going to fall also? it won't be fun out on the roads if the temps go below 32. what do you think Blizz?

They might go below freezing, but I still would think the main roads will be okay given the amount of salt from this morning. But there may be icy patches. I can't rule out some freezing drizzle also tonight.

Still 32.0F here with ice build up on the trees all day from drizzle and mist; pretty impressive. I still will never understand why the forecasters don't utilize the concept of cold air being denser than warm air.

I will say at this point that I think the storm next week has a better chance of causing rain in the big cities than going out to sea.

Quoting doum1982:IceCoast : Here in Quebec city, we had the same kind of problems in 2008 where we have received more than 500cm of snow during the winter... At home, two or three times a year, we get on the roof and take the snow out the roof... This way, we are sure we don't have any structural damage due to the heavy weight of the snow! You just need to be careful to don't fall from the roof!!!

Ya we tried getting some off this morning, but we have a very high and awkwardly shaped roof making it very dangerous. Roof rakes are sold out everywhere. I posted pictures a few days ago, and just took some more that I will post up soon. I've never seen anything like this, except the Blizzard of '78 my parents tell me.

Yeah, looking like a "Bust" for our big end of week snow storm! I hope things change, but it doesn't look good. Looks like that arctic high that's coming in will just force anything that forms off the Va. coast, and out to sea.

IceCoast : Here in Quebec city, we had the same kind of problems in 2008 where we have received more than 500cm of snow during the winter... At home, two or three times a year, we get on the roof and take the snow out the roof... This way, we are sure we don't have any structural damage due to the heavy weight of the snow! You just need to be careful to don't fall from the roof!!!

that is looking like a classic nc storm blockquote class='blogquote'>Quoting P451:12Z GFS (red) and NAM (Green) on the Thursday/Friday scenario. GFS actually has two systems following each other two days apart.

Pretty far off the coast and heading ENE does not favor anything significant for the Mid-Atlantic let alone inland.

Still, got to get these couple of systems out of the way first before the steering currents become more defined.

A dangerous situation here in Northeast MA. Moderate rain right now with snow and sleet mixed in with an air temp of 32.5 degrees and over 40" of snow on the ground. I am really hoping it doesn't start freezing to surfaces. Schools in my town are closed until at least tuesday due to 10 foot snowdrifts on the roofs, as well as several structural collapses in the area. I have several leaks in my house and it looks only to get worse.

Here's an article from a local paper describing the problems in the area. Link

As someone who snowboards on a daily basis i love the winter weather, but this isn't cool!

Quoting Blizzard92:Yep, always out early Saturday mornings. Temperatures will warm about freezing and conditions are much better to the south already, and they will be improving throughout the day. Precipitation will even be ending later today. No snow, just some plain rain/mist today.

thanks....so no problems with refreezing this evening? I am wondering about the drive coming back

you were out in this mess this morning? what do you expect for the rest of the day? I am supposed to go to a birthday party this evening in Bel Air, MD but not sure about the weather for latter tonight. any help with the weather would be appreciated

Yep, always out early Saturday mornings. Temperatures will warm about freezing and conditions are much better to the south already, and they will be improving throughout the day. Precipitation will even be ending later today. No snow, just some plain rain/mist today.

Quoting Blizzard92:Roads were a mess here this morning. Gosh it was terrible along my neighborhood around 6:30am with the roads just a sheet of ice. 31.0F here currently with about .1in of freezing rain on everything. Roads are now wet though.

I am still waiting for confirmation before I make an announcement. It is not that big of a deal, but will be interesting for everyone.

you are too funny Blizz about your annoucement.

you were out in this mess this morning? what do you expect for the rest of the day? I am supposed to go to a birthday party this evening in Bel Air, MD but not sure about the weather for latter tonight. any help with the weather would be appreciated

Roads were a mess here this morning. Gosh it was terrible along my neighborhood around 6:30am with the roads just a sheet of ice. 31.0F here currently with about .1in of freezing rain on everything. Roads are now wet though.

I am still waiting for confirmation before I make an announcement. It is not that big of a deal, but will be interesting for everyone.

DiMartino is calling for a "scattered snow showers" on Wednesday, wintry mix on Thursday, and scattered snow showers again on Friday. Sounds like he's not expecting very much, or it's just still very unclear as to what's going to happen. I hope you're right, TT and pittsburghnurse - I'd love another big snow before winter ends.

Quoting originalLT:Just a question, Blizz didn't tell us what his "surprise" was, did he? I didn't see it here, but maybe he mentioned something on Facebook, which I don't use.

I think it was the new photo which is a pretty cool update actually. I think it issued early for some of us hence ruining the little surprise. It might be what caused some of the old picture / new picture confusion for some of us.

It's too far out and too risky for any kind of reliable forecast and a good idea at this point to keep a low tone to it, but I'd like to know if Blizz has anything to say about it and if there's something I'm not seeing etc... Not too impressed with today's event or Monday/Tuesday's event for my location so I'm jumping over that for more interest later this week.

NWS for me...Wednesday: Mostly sunny, with a high near 33.

Wednesday Night: A chance of snow. Mostly cloudy, with a low around 19. Chance of precipitation is 40%.

Thursday: A chance of snow. Cloudy, with a high near 33. Chance of precipitation is 50%.

Thursday Night: A chance of snow showers. Partly cloudy, with a low around 13. Chance of precipitation is 30%.

Friday: Mostly sunny and breezy, with a high near 23.-------

Here are parts of the local discussion, which they haven't updated too much in quite a while. I pulled a few pieces out which I feel are still relevant...

Synopsis...A coastal storm may affect our area Wednesday night or Thursday but there is uncertainty with this low pressure system.

Long term /Wednesday through Friday/...Could be a significant but brief late season cold outbreak Thursday night through Friday night and still considerable uncertainty regarding a substantial western Atlantic low pressure system.

The 00z/4 ECMWF, its ensembles and indications from the 12z/4 Canadian Gem were followed regarding the system centered around Thursday. We have to note that the 12z European model (ecmwf) has come in with a slower and more easterly solution. It is sufficiently cold to raise what we know intuitively, I.E., That this forecast is very tenuous and that there is some possibility of a high impact event centered around Thursday.

No real problems around here this morning. No snow accumulation and a trace of frz rn accrual on the coldest of surfaces, roads just wet.

We're 4 1/2 days away from the Wednesday night - Friday morning coastal threat for the Mid-Atlantic / Northeast. Lot's of time to weed things out, but I think the area is sitting in a pretty good position for a large event at this point, especially for the eastern half. Everything has trended just southeast of benchmark so I believe the inland runner solution is becoming more unlikely. At this point, things are still too far east to give the typically more favored well-inland areas a high impact event and models are ranging from nothing to a light snow event there. Right now models are ranging from a light to heavy snow event for the I-95 corridor and southeast.

It's my opinion that this is an optimal track forecast (just southeast of benchmark) for much of our area in the 4 to 6 day range as it is true that you don't want to be in the bullseye more than two days out as those track solutions will almost always change. If things were trending westward at this point towards an inland runner than hopes would be fading for a heavy snow event for eastern and southern sections.

Track location in the range just before you hit the stretch is somewhat comparable to horse racing. In horse racing, as they round the last corner before hitting the stretch, your optimal positioning is to be around the #3 spot. Being in the #1 spot at that point often results in a fade into the middle or back of the pack by the time you reach the finish line. You also don't want to be in the #6 spot or worse because chances are that there is not enough time to take the lead by the time you reach the finish line. It is also more rare that a horse takes the #1 position from wire to wire. Lol, what the hell am I talking about?

Summing up, I believe parts of our area are sitting pretty for a high impact event Wednesday night into Friday morning.

At the moment, the ECMFW initiates stronger and earlier phasing than does the GFS.

Rain just started. Air temp 33.6. Hopefully its just straight rain. Low of 25 expected. Sounds sloppy. Wish I could get cross my back yard to the bird feeders. The 2-3" ice rink that's formed most of the whole back yard makes that impossible. I wonder when that thing will melt.

Just got home from an aborted trip to the high school. Somehow we didn't get the email telling us that musical practice was canceled. Within a block of my house there were three accidents and the police directed me away from Nyes Road to the back roads. Many cars were slid off of the road. Once we got back to the main roads they are just wet but those back roads are way worse than the other day. As we were trying to get back up the driveway (I chose to crunch up the snow banks) the rain was really picking up and still freezing.

Quoting TheRasberryPatch:oh it's so nice to wake up to a nice glaze of ice everywhere. freezing rain since early this morning and you can not walk outside. it was sad watching my dog trying to walk a couple of feet from the garage to the driveway to a stretch of grass to do his business.

i hope this weather leaves early today and not continue into the evening

Blah! Have the same conditions forecasted here. Winter is now like a guest who has overstayed his welcome before he's due to check out.

Just a light dusting of snow here, the precip. has stopped for now but more moving towards me on radar, probably when it gets here it will be sleet or freezing rain then rain by late morning. 30F. Baro. is 29.98" and falling slowly. Very light winds out of the East right now. LT Stamford CT.-- Got down to 16F over night. we had good radiational cooling last night, it was clear untill about 1AM.

oh it's so nice to wake up to a nice glaze of ice everywhere. freezing rain since early this morning and you can not walk outside. it was sad watching my dog trying to walk a couple of feet from the garage to the driveway to a stretch of grass to do his business.

i hope this weather leaves early today and not continue into the evening

Just discovered your blog 2 weeks ago. Now something to look forward to daily. Enjoy your professional quality work and responses. Looking forward to your analysis of this coming storm. Do you 'work the tropics' as well during hurricane season or are continental systems your specialty.